The isomerization of xylene feedstocks is a conventional procedure in petroleum refining operations. Most such feedstocks contain the isomers ortho-, meta-, and para-xylene, and ethylbenzene together with small quantities of other aromatic compounds and saturated hydrocarbons. Xylene isomerization normally is carried out as an intermediate function in a so-called "xylene loop" in which a desired xylene isomer (usually para-xylene but in some cases also ortho-xylene) is withdrawn from a mixture of the isomers found in a process stream such as the output from a reforming unit. The remainder of the process to stream is used as a feed stock for the isomerization unit. The output from the isomerization unit is recycled and mixed with fresh charge to the xylene loop.
While ortho-xylene can be separated from the other xylene isomers and ethylbenzene by fractional distillation para-xylene which has a boiling point about 1.degree. C. below meta-xylene and about 2.degree. C. above ethylbenzene is normally separated in the xylene loop by crystallization or selective adsorption. Typically, the para-xylene content is reduced by the crystallization or selective adsorption step to less than 10 and preferably less than 5 weight percent. The feedstream with the para-xylene thus extracted is applied to the isomerization reactor where isomerization of the ortho- and meta-xylenes results in a product in which the para-xylene is at approximately equilibrium concentration. The procedures and reactions involved in xylene isomerization are described in greater detail in Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, John Wiley & Sons 1984, "Xylenes and Ethylbenzene," Vol. 24, pages 709-744, to which reference is made for a more complete description of this process.
As described in the above referenced Kirk-Othmer article starting at page 732 under the heading "Zeolite-Based Xylene Isomerization," the isomerization of xylenes employing ZSM-5 type zeolite catalyst has been widely practiced. Various procedures employing such catalysts are known in the prior art. For example, U.S. Pat. No. 3,856,872 to Morrison discloses xylene isomerization using aluminosilicalite zeolite catalyst, specifically a ZSM-5 type, a ZSM-12, or a ZSM-21 catalyst. The xylene feedstream is applied, along with the hydrogen cofeed, to the isomerization reactor which is operated at a temperature of 550.degree.-900.degree. F. and a pressure of 150-300 psi. The Morrison isomerization step decreases the ethylbenzene and meta-xylene content of the process stream but increases the ortho-xylene along with the para-xylene content. Thus, Morrison discloses employing a nickel promoted ZSM-5 type catalyst to isomerize a feedstream containing about 17% ethylbenzene, 11% para-xylene, 65% meta-xylene and 7% ortho-xylene. The product contains about 4-6% ethylbenzene, 16-18% para-xylene, 40-43% meta-xylene or about 17-18% ortho-xylene. Somewhat similar results were achieved using nickel promoted ZSM-12 or ZSM platinum promoted ZSM-12 or nickel promoted ZSM-21 except that a somewhat less ethylbenzene reduction was observed.
U.S. Pat. No. 4,163,028 to Tabak et al discloses a xylene isomerization process employing a high silica/alumina ratio zeolite identified as ZSM-5, ZSM-11, ZSM-12, ZSM-38, ZSM-35 and ZSM-5 co-crystallized with a metal such as platinum. Typical catalysts are described as ZSM-5 having silica/alumina ratio of about 500 to 3000 or greater. The patent to Tabak et al describes the above-mentioned Morrison process as involving disproportionation of ethylbenzene and disproportionation and ethylation of xylenes. In the Tabak procedure, the isomerization reaction is carried out at a temperature above 800.degree. F. in order to diminish the ethylbenzene concentration by dealkylation, as contrasted with the disproportionation reaction of Morrison. The Tabak et al procedure is similar to that of Morrison in that the ortho-xylene content in the product is increased, although in most cases to a somewhat lesser extent than is the case in Morrison.
U.S. Pat. No. 4,159,282 to Olsen et al discloses xylene isomerization carried out over a crystalline aluminosilicate zeolite having a crystal size of at least one micron. This catalyst can be employed alone or in combination with a crystalline aluminosilicate zeolite having a crystal size of less than one micron. The preferred crystalline aluminosilicates are said to be ZSM-5, ZSM-11, ZSM-12, ZSM-38 and ZSM-35. Olsen et al specifically discloses xylene isomerization carried out over a ZSM-5 zeolite and disclosed that a ZSM-5 zeolite having an average crystal size of about 2 microns is more xylene selective (the ratio of relative ethylbenzene loss to relative xylenes loss is greater) than a ZSM-5 catalyst having a crystal size of about 0.5 micron. The enhanced para-xylene product of the Olsen process has a reduced ethylbenzene content and a modestly decreased ortho-xylene content. Olsen et al states that the isomerization reactor may be operated to produce para-xylene as the sole product with a typical charge of 20 weight percent ethylbenzene, 51% meta-xylene, 9 weight percent para-xylene and 20 weight percent ortho-xylene, although the experimental data in the reference do not appear to support this statement.
Additional xylene isomerization processes employing aluminosilicate zeolites are disclosed in U.S. Pat. Nos. 4,188,282 and 4,218,573 to Tabak et al. The catalysts may be ZSM-5 type zeolites having silica/alumina ratios greater than 200 and preferably greater than 500. In each of these references, the conversion temperature is above 800.degree. F. The ethylbenzene content in the isomerization feed is decreased by a mechanism which is said to involve little, if any, disproportionation. In each case, the ortho-xylene content of the product is increased substantially along with the para-xylene content.